The use of fiberoptic networks is increasing due to the high bandwidth provided by such networks for transporting data, voice, and video traffic. Large switches would help to accommodate the switching needs of many of the larger fiberoptic networks, especially the high-capacity fiber backbones.
One disadvantage of certain prior art optical switches is that although optical signals can propagate almost losslessly while confined in optical fiber, the size of certain prior art optical switches is typically limited by diffraction of optical beams as they propagate through free space inside the switches. Moreover, large optical switching devices can be difficult to construct given the large number of optical cables and beams and complex associated electrical connection issues. In short, large optical switches can be costly and unwieldy.
Various types of non-optical electrical switch fabrics have been used in the prior art for telephony and network applications. One of the simplest structures has been the crossbar switch. One problem with the crossbar switch is the quadratic growth of crosspoints as the switch gets larger, which can result in far more cross-points than necessary to create all possible permutation connections. For a permutation switch, connections between input and output ports are point to point—neither one-to-many nor many-to-one connections are permitted.
To avoid the problem of excess crosspoints found in a single large switch, techniques have been developed for cascading small electrical switches into a multistage switch fabric in order to make large electrical permutation switches.
Permutation switches can be classified in terms of their blocking characteristics. On a switch, requests for connection establishment and termination can occur at random points in time. A permutation switch is rearrangeable or rearrangeably nonblocking if there exists a set of paths through the switch fabric that realizes each of any possible connection states. The rearrangeable aspect means that it may be necessary to rearrange currently active connections to support a request for a new connection between a pair of idle input and output ports. Problems with rearrangeable nonblocking switches include the fact that the required device settings to route connections through the switch are not determined easily and that connections in progress may have to be interrupted momentarily while rerouting takes place to handle the new connections.
Wide-sense nonblocking networks or switches are those that can realize any connection pattern without rearranging active connections provided that the correct rule is used for routing each new connection through the switch fabric.
Strict-sense nonblocking networks or switches require no rearrangement of active connections and no complex routing algorithms. New connection requests are allowed to use any free path in the switch. Strict-sense nonblocking switching fabrics (also referred to as strictly nonblocking switches) typically require more hardware than wide-sense nonblocking and rearrangable switching fabrics, but avoid connection disruption and provide simplicity of routing.
One type of cascaded permutation switch topology is a Clos switch fabric, also referred to as a Clos network, a Clos switch matrix, or a Clos switch. Various Clos switch configurations can constructed. For example, some Clos switch fabrics can be strict-sense nonblocking, other Clos switch fabrics can be wide-sense nonblocking, and others can be blocking. The blocking configurations are less useful, given that some combinations of input and output connections cannot be made.
FIG. 1 shows a three-stage Clos switch fabric that is strict-sense nonblocking, meaning that any input can be routed to any output at any time. The Clos switch fabric of FIG. 1 has N inputs, N outputs, K input stage switches, 2p−1 center stage switches, and K output stage switches. Each input stage switch has p inputs and 2p−1 outputs. Each center stage switch has K inputs and K outputs. Each output stage switch has 2p−1 inputs and p outputs.
One disadvantage of the strict-sense nonblocking Clos switch fabric of FIG. 1 is the lack of redundancy in switch connections. Redundancy is a desirable characteristic in a switch fabric because redundancy helps to permit rerouting in the event of a failure, the use of extra paths for test purposes during switch operation, and switch reconfiguration during switch operation.